Sustainable

A practice is sustainable if it can be continued indefinitely within a system.

Indefinitely? Without limit, except as posed by external events truly beyond the control of those performing the practice that change the system.

Is there a limit beyond which it is not possible to continue? Assuming planet earth as the system, the sun is expected to put an upper limit on any biosphere after several billion years. Other events beyond our control that make life impossible on this planet may or may not occur. For purposes of this discussion sustainable civilization is able to continue on this planet while the sun provides a suitable environment i.e., for billions of years.

Wikipedia's articles on sustain and sustainability discuss various concepts and understandings of the term.

More sustainable?

The term "more sustainable" is commonly used even though it is misleading. "More sustainable" suggests a practice that is suitable for a longer period than previous practices. Such a longer period offers more time to implement practices that can indeed be continued. That is worthwhile only as long as it does not divert attention from the effort to come up with a sustainable solution. No matter how much effort is spent on finding "more sustainable" improvements, someone will eventually have to come up with a solution that can be continued.

"More sustainable" improvements so far seem to postpone serious problems by no more than one generation (that is a rough assumption, but I can't easily think of examples that do much better). That is a far cry from the millions of generations that may be possible.

Nature shows that it can be done. Every civilization should try. Most have failed so far. We know more now. Admitting that "more sustainable" is not sustainable may help.